Reorientation Time Research Articles

Electron Detachment and Relaxation of OH-(aq)

Femtosecond transient absorption spectroscopy is used to study the primary reaction dynamics of photoinduced electron detachment of the hydroxide ion in water, OH- (aq). The electron is detached by excitation of OH- (aq) to the charge-transfer-to-solvent (CTTS) state at 200 nm. The subsequent relaxation processes are probed in the spectral range from 193 to 800 nm with femtosecond time resolution. We determine both the time-dependent quantum yields of OH- (aq), OH(aq), and e-(aq), and we observe a transient spectral signature which is assigned to relaxation of hot (OH-)* ions formed via solvent-assisted conversion of the excited CTTS state to OH-. The primary quantum yield of OH(aq) is 65 +/- 5%, while recombination with e-(aq) reduces the yield to 34% after 5 ps and 12% after 200 ps. The yield of hot (OH-)* ions is 35 +/- 5%. Rotational anisotropy measurements of OH- (aq) and OH(aq) indicate a reorientation time for OH- (aq) of 1.9 ps, while no rotational anisotropy is resolved for the OH(aq) radical within our time resolution of 0.3 ps. This is consistent with the notion that OH(aq) radicals formed after electron detachment are only weakly bound to the hydrogen bond network of water. The assignment of the experimental data is supported by a series of electronic structure calculations of simple complexes of OH- (H(2)O)(n).

Rotational diffusion of ionic and neutral solutes in mixed micelles: Effect of surfactant to block copolymer mole ratio on solute rotation

Rotational diffusion of an ionic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) has been investigated in aqueous mixtures of cetyltrimethylammonium chloride (CTAC) and poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123). The purpose of this work is to understand how an increase in the mole ratio of surfactant to block copolymer from low to high influences the dynamics of ionic and neutral solute molecules. The variation in the mole ratio of CTAC to P123 from low to high has resulted in a drastic increase in the average reorientation time of rhodamine 110. In contrast, an exactly opposite trend has been noticed in the case of DMDPP. In the low mole ratio regime, rhodamine 110 and DMDPP are located at the interface and palisade layer, respectively, of P123 micelle-CTAC complexes. On the other hand, in the high mole ratio regime, both the probes are located in the Stern layer of CTAC-P123 complexes. The enhancement in the average reorientation time of rhodamine 110 with an increase in the mole ratio of surfactant to block copolymer has been rationalized on the basis of formation of rhodamine 110-Cl ion pair, which in turn associates with the cationic head groups of CTAC-P123 complexes. The observed decrease in the average reorientation time of DMDPP with an increase in the mole ratio of CTAC to P123 is a consequence of lower microviscosity of the Stern layer of CTAC-P123 complexes compared to the palisade layer of P123 micelle-CTAC complexes.

Terahertz characteristics of electrolytes in aqueous Luria-Bertani media

We measured the optical constants of aqueous biomaterial mixtures with various electrolyte concentrations using terahertz time-domain spectroscopy. The mixtures were divided into water and other electrolyte parts in mass fractions for analysis. The optical constants of the electrolyte, excluding water, were obtained by applying the ideal mixture equation, and the power absorption of the electrolyte was observed to be larger than that of water above 1THz. Data from the measurement were fitted with the modified double Debye model, and the reorientation and hydrogen-bond formation decomposition times were found to decrease as the electrolyte concentration increased.

Rotational dynamics of UVITEX-OB in alkanes, alcohols and binary mixtures

Steady-state and time resolved fluorescence anisotropy measurements were carried out to study the rotational diffusion dynamics of UVITEX-OB (U-OB) in series of alcohols, alkanes and binary mixtures of toluene and butanol at room temperature. The experimentally measured rotational reorientation times were compared with those estimated by the hydrodynamic and molecular models developed for microscopic friction. The experimental results are in good agreement with theoretical slip hydrodynamics and a deviation towards subslip behavior is noted. Also a faster rotation of the probe in binary mixture of toluene and butanol is noted as compared to that in alcohols and alkanes.

Adsorption of Acridine Orange at a C8,18/Water/Acetonitrile Interface

Fully atomistic simulations are used to characterize the molecular dynamics (MD) of acridine orange (3,6-dimethylaminoacridine) at a chromatographic interface. Multiple 1 ns MD simulations were performed for acridine orange at the interface between three different acetonitrile/water mixtures (0/100, 20/80, and 50/50) with C8 and C18 alkyl chains. The diffusion coefficient, D, of acridine orange in pure solvent was found to be 4 times smaller at the water/C18 interface (D = 0.022 x 10(-4) cm2/s) than in bulk water (D = 0.087 x 10(-4) cm2/s), in qualitative agreement with experiment. Rotational reorientation times were 20 and 700 ps, which also agree favorably with the measured time scales of 130 and 740 ps. Contrary to experiment, the simulations found that for increasing surface coverage, the diffusion coefficient for acridine decreased. Detailed analysis of the solvent structure showed that the transport properties of acridine were primarily governed by the solvent distribution above the functionalized surface. The solvent structure, in turn, was largely determined by the surface consisting of the silica layer, the alkyl chains, and their functionalization.

Excited-State Dynamics of Donor−Acceptor Bridged Systems Containing a Boron−Dipyrromethene Chromophore: Interplay between Charge Separation and Reorientational Motion

The excited-state dynamics of a series of electron donor-acceptor bridged systems (DABS) consisting of a boron-dipyrromethene chromophore covalently linked to a dinitro-substituted triptycene has been investigated using femtosecond time-resolved spectroscopy. The chromophores differ by the number of bromine atom substituents. The fluorescence lifetime of the DABS without any bromine atom is strongly reduced when going from toluene to polar solvents, this shortening being already present in chloroform. This effect is about 10 times weaker with a single bromine atom and negligible with two bromine atoms on the chromophore. The excited-state lifetime shortening is ascribed to a charge transfer from the excited chromophore to a nitrobenzene moiety, the driving force of this process depending on the number of bromine substituents. The occurrence of this process is further confirmed by the investigation of the excited-state dynamics of the chromophore alone in pure nitrobenzene. Surprisingly, no correlation between the charge separation time constant and the dielectric properties of the solvents could be observed. However, a good correlation between the charge separation time constant and the diffusional reorientation time of the chromophore alone, measured by fluorescence anisotropy, was found. Quantum chemistry calculations suggest that quasi-free rotation about the single bond linking the chromophore to the triptycene moiety permits a sufficient coupling of the donor and the acceptor to ensure efficient charge separation. The charge separation dynamics in these molecules is thus controlled by the reorientational motion of the donor relative to the acceptor.

Phase transitions and NH 3 motions in [Zn(NH 3) 4](ClO 4) 2 studied by incoherent neutron scattering and 1H NMR methods

Quasielastic and inelastic incoherent neutron scattering (QENS and IINS) and the analysis of the second moment ( M 2) of proton magnetic resonance ( 1H NMR) line for [Zn(NH 3) 4](ClO 4) 2 crystals in the function of temperature are reported. The QENS peak registered at 75 K and also at higher temperatures shows distinct broadening, which is typical for dynamically, orientationally disordered crystals (ODIC). Reorientational motion of NH 3 ligands can be well described by a model of 120° proton jumps around a 3-fold axis of Zn–N bonds on a picoseconds time scale. The NH 3 do not drastically change either the jump rate or the character of their reorientational motion at the phase transitions at: T C1 ≈ 271 K and T C2 ≈ 164 K. However, it undergoes a very small change of the activation energy E a (120°) value for such a motion at T C2. The estimated E a value equals 2.5 kJ mol −1 for high and intermediate temperature phases and 1.7 kJ mol −1 for the low temperature phase. 1H NMR studies revealed that NH 3 ligands perform reorientation around the 3-fold symmetry axis close to 90 K when the second moment of the H NMR line exhibits the plateau of M 2 (reorientational correlation time is less than reciprocal of the line width). The estimated E a(120°) ≈ 3 kJ mol −1 is in quite good agreement with that calculated from the QENS measurements. Above the phase transition at T C2, the anisotropic reorientation of the whole [Zn(NH 3) 4] 2+ cation around the 3-fold symmetry axis is set in motion with a frequency in the order of several kHz (the 1H NMR line width).

Structure and internal dynamics of a side chain liquid crystalline polymer in various phases by molecular dynamics simulations: A step towards coarse graining

Side chain liquid crystalline polymer with relatively long spacer was modeled on a semiatomistic level and studied in different liquid crystalline phases with the aid of molecular dynamics simulations. Well equilibrated isotropic, polydomain smectic and monodomain smectic phases were studied for their structural and dynamic properties. Particular emphasis was given to the analysis on a coarse-grained level, where backbones, side chains, and mesogens were considered in terms of their equivalent ellipsoids. The authors found that the liquid crystalline phase had a minor influence on the metrics of these objects but affected essentially their translational and orientational order. In the monodomain smectic phase, mesogens, backbones, and side chains are confined spatially. Their diffusion and shape dynamics are frozen along the mesogen director (the one-dimensional solidification) and the reorientation times increase by one to one-and-half orders of magnitude. In this phase, besides obvious orientational order of mesogens and side chains, a stable detectable order of the backbones was also observed. The backbone director is confined in the plane perpendicular to the mesogen director and constantly changes its orientation within this plane. The backbone diffusion in these planes is of the same range as in the polydomain smectic phase at the same temperature. A detailed analysis of the process of field-induced growth of the smectic phase was performed. The study revealed properties of liquid crystalline polymers that may enable their future fully coarse-grained modeling.

Rotational Diffusion of Organic Solutes in Surfactant−Block Copolymer Micelles: Role of Electrostatic Interactions and Micellar Hydration

Rotational diffusion of a cationic solute rhodamine 110 and a neutral solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole, DMDPP has been examined in the surfactant-block copolymer system of sodium dodecyl sulfate (SDS) and poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123). In this study, the mole ratio of SDS to P123 was varied from 0 to 5 in steps of one unit, to investigate the role of electrostatic interactions and micellar hydration on solute rotation. It has been noticed that there is a significant enhancement in the average reorientation time of rhodamine 110, when [SDS]/[P123] increased from 0 to 1. This has been rationalized on the basis of migration of rhodamine 110 from the interfacial region of P123 micelles to the palisade layer (corona region) due to the electrostatic interaction with negatively charged head groups of SDS, whose tails are embedded in the polypropylene oxide core. Further increase in the mole ratio of SDS to P123 has resulted in only a marginal decrease in the average reorientation time of rhodamine 110, which is probably due to the solute molecule experiencing a microenvironment similar to the interfacial region of SDS micelles. In contrast, a gradual decrease has been observed in the average reorientation time of DMDPP with [SDS]/[P123], which is due to the increase in hydration levels in the palisade layer (corona region) of the micelle. These explanations are consistent with the structure of the SDS-P123 micellar system that has been deduced from neutron scattering and viscosity measurements recently.

Local Polarity and Microviscosity in the Hydrophobic Cores of Amphiphilic Star-like and Scorpion-like Macromolecules

Fluorescence spectroscopy is used to probe the polarity and local friction in aqueous solutions of three amphiphilic polymers. Aggregate core polarity and local friction on the nanoscale are observed by measuring the steady-state and time-resolved fluorescence spectroscopy of coumarin 153 (C153) in three amphiphilic polymers as a model for hydrophobic drug encapsulation. Three solutions of polymers are studied: one amphiphilic scorpion-like macromolecule (AScM), M12P5, and two amphiphilic star-like macromolecules (ASMs), NC12P5 and NC6P5. Both ASMs and AScMs consist of hydrophobic segments of acylated mucic acid and hydrophilic poly(ethylene glycol) chains. In aqueous solution, the scorpion-like M12P5 forms micellar aggregates through hydrophobic interactions, while the star-like polymers are covalently bonded macromolecules that form higher order aggregates. Micelle and aggregate sizes are compared using dynamic light scattering measurements. A shift in the steady-state emission wavelength of C153 indicates distinct differences in polarity between the core environments of the scorpion-like polymer micelles and those of the two star-like polymers. The two reorientation time constants from C153 fluorescence anisotropies show that the hydrophobic cores of M12P5 micelles are relatively flexible. The covalently linked, rigid cores of the two star-like macromolecule nanocarriers, NC6P5 and NC12P5, show a third reorientation time constant for which the local friction of the dye is remarkably higher.

Tl(I)‐the strongest structure‐breaking metal ion in water? A quantum mechanical/molecular mechanical simulation study

Structural and dynamical properties of the Tl(I) ion in dilute aqueous solution have been investigated by ab initio quantum mechanics in combination with molecular mechanics. The first shell plus a part of the second shell were treated by quantum mechanics at Hartree-Fock level, the rest of the system was described by an ab initio constructed potential. The radial distribution functions indicate two different bond lengths (2.79 and 3.16 A) in the first hydration shell, in good agreement with large-angle X-ray scattering and extended X-ray absorption fine structure spectroscopy results. The average first shell coordination number was found as 5.9, and several other structural parameters such as coordination number distributions, angular distribution functions, and tilt- and theta-angle distributions were evaluated. The ion-ligand vibration spectrum and reorientational times were obtained via velocity auto correlation functions. The Tl-O stretching force constant is very weak with 5.0 N m(-1). During the simulation, numerous water exchange processes took place between first and second hydration shell and between second shell and bulk. The mean ligand residence times for the first and second shell were determined as 1.3 and 1.5 ps, respectively, indicating Tl(I) to be a typical "structure-breaker". The calculated hydration energy of -84 +/- 16 kcal mol(-1) agrees well with the experimental value of -81 kcal mol(-1). All data obtained for structure and dynamics of hydrated Tl(I) characterize this ion as a very special case among all monovalent metal ions, being the most potent "structure-breaker", but at the same time forming a distinct second hydration shell and thus having a far-reaching influence on the solvent structure.

Dynamics of supercooled water in various mesopore sizes

Double-quantum-filtered NMR and T(1) inversion-recovery spectroscopy were employed to exploit the temperature-dependent dynamics of D(2)O confined in MCM-41. Samples with three pore sizes of 1.58, 2.03, and 2.34 nm and two D(2)O contents were investigated. The reorientation correlation times of confined D(2)O in variously sized pores exhibit different temperature dependencies. The results reveal that the D(2)O molecules at fast motion site remain mobile below approximately 225 K and a liquid-liquid phase transition occurs around this temperature for all samples studied. This temperature is thought to be unreachable for supercooled D(2)O. Particularly, in 20 wt % D(2)O loaded samples with pore diameters of 1.58 and 2.03 nm, the reorientational correlation times of D(2)O at fast motion site exhibit Arrhenius behavior between 225 and 290 K, while other samples show power law dependency. Thus, a liquid phase of the fragile type in bigger pores changes to the strong type in samples with smaller pores.

New CD derivatives as self-assembling contrast agents for magnetic resonance imaging (MRI)

A way to improve Magnetic Resonance Imaging is to deliver a larger number of Imaging Probe units to the target site. Aiming at this objective, we prepared a self-assembling system consisting of: 1) a β-cyclodextrin (β-CD) bearing a covalently bonded Gd complex (DTPA-Lys); 2) a polypeptide containing a high percentage of tyrosine residues (PLT); 3) a second β-CD derivative bearing a covalently bonded peptide vector (CCK8) that can recognize a specific cell-membrane receptor. Both β-CD derivatives can form stable inclusion complexes with the aromatic moieties of the polypeptide. The formation of a supramolecular adduct having a long reorientational correlation time entailed a marked relaxivity increase (per Gd3+ ion), which recommends it as a promising model for detail enhancement procedures at the target site. Out of three different synthetic pathways that could be used for binding a CD to DTPA, the most convenient one involved a micro-wave(MW)-assisted Mannich aminomethylation of a monopropargyl β-CD by the primary amino group of t-butyl-DTPA-Lys.

Evolution of Intermolecular Structure and Dynamics in Supercritical Carbon Dioxide with Pressure: An ab Initio Molecular Dynamics Study

The effect of pressure on supercritical carbon dioxide (scCO2) has been characterized by using Car-Parrinello molecular dynamics simulations. Structural and dynamical properties along an isotherm of 318.15 K and at pressures ranging from 190 to 5000 bar have been obtained. Intermolecular pair correlation functions and three-dimensional atomic probability density map calculations indicate that the local environment of a central CO2 molecule becomes more structured with increasing pressure. The closest neighbors are predominantly oriented in a distorted T-shaped geometry while neighbors separated by larger distances are likely oriented in a slipped parallel arrangement. The structure of scCO2 at high densities has been compared with that of crystalline CO2. The probability distributions of intramolecular distances narrow down with increasing pressure. A marginal but non-negligible effect of pressure on the instantaneous intramolecular OCO angle is observed, lending credence to the idea that intermolecular interactions between CO2 molecules in an inhomogeneous near neighbor environment could contribute to the observed instantaneous molecular dipole moment. The extent of deviation from a perfect linear geometry of the carbon dioxide molecule decreases with increasing pressure. Time constants derived from reorientational time correlation functions of the molecular backbone compare well with experimental data. Within the range of thermodynamic conditions explored here, no significant changes are observed in the frequencies of intramolecular vibrational modes. However, a blue shift is observed in the low-frequency cage rattling mode with increasing pressure.

Rotational Diffusion of an Ionic Solute in Polymorphic Environments of a Block Copolymer: Influence of Interfacial Friction on Solute Rotation

In an attempt to understand the role of interfacial friction on solute rotation, fluorescence anisotropy decays of a cationic solute, rhodamine 110, have been measured in polymorphic environments of a triblock copolymer, (PEO)20-(PPO)70-(PEO)20 (P123) (PEO = poly(ethylene oxide), PPO = poly(propylene oxide)). It has been noticed that even though rhodamine 110 is located in the interfacial region of the micelles, sol-gel transition does not significantly influence its rotation. Micelle-micelle entanglement, which is responsible for gelation, persists even in the micellar solution phase, perhaps to a lesser degree, and this entanglement is responsible for the observed behavior. This hypothesis has been substantiated by undertaking concentration-dependent studies in which it is shown that the reorientation time of the solute increases with an increase in the micellar concentration. In the case of reverse micelles, it has been observed that an enhancement in the water content facilitates solute rotation, which has been rationalized on the basis of solute migration from the hydrated poly(ethylene oxide) region to the poly(ethylene oxide)-water interface within the core.

Density dependence of optical Kerr lens signals in neat N 2O and CO 2

Optical Kerr lens spectroscopy has been employed to study the reorientation dynamics of N 2O and CO 2 at the reduced densities 0.8–2.0 and different temperatures in a newly constructed high pressure cell. The transient birefringence induced in the medium by an ultrashort (1 1 0 fs) non-resonant pump beam at 400 nm was monitored by a second, time-delayed non-resonant probe beam at 800 nm. The transient change of the probe beam diameter was detected in the far field using a small-area photodiode. Measurements employing parallel and perpendicular polarization allowed us to extract the nuclear and electronic components of the Kerr effect, where the latter one was by far weaker for both CO 2 and N 2O. Accurate time constants for the reorientation dynamics of the CO 2 and N 2O molecules were extracted from the tail of the nuclear Kerr response. In the case of CO 2 the time constants τ reorientation were between 210 and 280 fs and in good agreement with previous data from NMR and OHD-OKE experiments as well as MD simulations. Reorientation time constants for N 2O were in the range between 200 and 340 fs, and thus very similar to CO 2. In the density range studied, τ reorientation increases with increasing viscosity and decreasing temperature, in agreement with Stokes–Einstein–Debye diffusive models.

Phase transitions and molecular motions in [Zn(NH 3) 4](BF 4) 2 studied by nuclear magnetic resonance, infrared and Raman spectroscopy

Middle infrared absorption, Raman scattering and proton magnetic resonance relaxation measurements were performed for [Zn(NH 3) 4](BF 4) in order to establish relationship between the observed phase transitions and reorientational motions of the NH 3 ligands and BF 4 − anions. The temperature dependence of spin-lattice relaxation time ( T 1( 1H)) and of the full width at half maximum (FWHM) of the bands connected with ρ r(NH 3), ν 2(BF 4) and ν 4(BF 4) modes in the infrared and in the Raman spectra have shown that in the high temperature phase of [Zn(NH 3) 4](BF 4) 2 all molecular groups perform the following stochastic reorientational motions: fast ( τ R ≈10 −12 s) 120° flips of NH 3 ligands about three-fold axis, fast isotropic reorientation of BF 4 − anions and slow ( τ R ≈10 −4 s) isotropic reorientation (“tumbling”) of the whole [Zn(NH 3) 4] 2+ cation. Mean values of the activation energies for uniaxial reorientation of NH 3 and isotropic reorientation of BF 4 − at phases I and II are ca. 3 kJ mol −1 and ca. 5 kJ mol −1, respectively. At phases III and IV the activation energies values for uniaxial reorientation of both NH 3 and of BF 4 − equal to ca. 7 kJ mol −1. Nearly the same values of the activation energies, as well as of the reorientational correlation times, at phases III and IV well explain existence of the coupling between reorientational motions of NH 3 and BF 4 −. Splitting some of the infrared bands at T C2=117 K suggests reducing of crystal symmetry at this phase transition. Sudden narrowing of the bands connected with ν 2(BF 4), ν 4(BF 4) and ρ r(NH 3) modes at T C3=101 K implies slowing down ( τ R ⪡10 −10 s) of the fast uniaxial reorientational motions of the BF 4 − anions and NH 3 ligands at this phase transition.

Molecular Dynamics Simulation of Liquid Water Confined inside Graphite Channels: Dielectric and Dynamical Properties

Electric and dielectric properties and microscopic dynamics of liquid water confined between graphite slabs are analyzed by means of molecular dynamics simulations for several graphite-graphite separations at ambient conditions. The electric potential across the interface shows oscillations due to water layering, and the overall potential drop is about -0.28 V. The total dielectric constant is larger than the corresponding value for the bulklike internal region of the system. This is mainly due to the preferential orientations of water nearest the graphite walls. Estimation of the capacitance of the system is reported, indicating large variations for the different adsorption layers. The main trend observed concerning water diffusion is 2-fold: on one hand, the overall diffusion of water is markedly smaller for the closest graphite-graphite separations, and on the other hand, water molecules diffuse in interfaces slightly slower than those in the bulklike internal areas. Molecular reorientational times are generally larger than those corresponding to those of unconstrained bulk water. The analysis of spectral densities revealed significant spectral shifts, compared to the bands in unconstrained water, in different frequency regions, and associated to confinement effects. These findings are important because of the scarce information available from experimental, theoretical, and computer simulation research into the dielectric and dynamical properties of confined water.

Rotational Diffusion and Solvatochromic Correlation of Coumarin 6 Laser Dye

Rotational diffusion of coumarin 6 (C6) laser dye has been examined in n-decane and methanol as a function of temperature. The rotational reorientation of this probe has been measured in these solvents. It is observed that the decrease in viscosity of the solution is responsible for the decrease in the rotational relaxation time of the probe molecule. The molecule C6 has long reorientation times in n-decane solvent as compared to methanol over all temperatures. It is found that the coumarin 6 rotates slower in n-decane than in methanol especially at higher values of viscosity over temperature. Two methods are chosen to determine the ground state and excited state dipole moments. The change in dipole moments is estimated from Bakhshiev-Chamma-Viallet equations and, the ground and excited state dipole moments from Kawski et al. equations, by using the variations of the Stokes shifts with the dielectric constant and refractive index of the solvent. Our results are quite reliable which are solvatochromic correlation obtained using solvent polarity functions. The reported results show that excited state dipole moment is greater than ground state dipole moment, which indicates that the excited state is more polar than the ground state.

Influence of Longitudinal Electric Field on Time Characteristics of Freedericksz Transition

Electrooptic properties of liquid crystal cells with semiconductor substrate are studied. A DC electric field has been applied along the semiconductor substrate. It is shown, that presence of such field results both in reduction of Freedericksz threshold and in essential reduction of characteristic reorientation and relaxation times. For more understanding of the mechanism of current flow through the liquid crystal – semiconductor interface, the Volt-Ampere (V-I) and spectral characteristics of photocurrent for such system have been investigated.